Photovoltaic energy is becoming a very significant power source for several reasons. Fossil fuels are becoming scarcer, and hence more expensive, every day. Furthermore, the burning of fossil fuels releases pollutants, including greenhouse gases which contribute to problems of global warming. Also, recent events have raised questions as to the safety and cost-effectiveness of nuclear power. For these reasons, traditional energy sources have become far less attractive. Photovoltaic energy, on the other hand, is inherently non-polluting, safe and silent. In addition, recent advances in photovoltaic technology have significantly increased the efficiency, and decreased the cost, of such device.
For example, it is now possible to manufacture large area silicon and/or germanium alloy materials which manifest electrical, optical, chemical, and physical properties equivalent, and in many instances superior to, their single crystalline counterparts. Layers of such alloys can be economically deposited at high speed over relatively large areas and in a variety of stacked configurations. Such alloys readily lend themselves to the manufacture of low cost photovoltaic devices. Examples of particular fluorinated semiconductor alloy materials having significant utility in fabrication of photovoltaic devices are described in U.S. Pat. No. 4,226,898 and U.S. Pat. No. 4,217,374, both invented by Ovshinsky et al, the disclosures of which are incorporated herein by reference.
In a typical large area photovoltaic device, a number of current-collecting structures are employed to convey photo-generated current to a terminal or other collection point. As used herein, these various structures will be referred to as "current-collecting grids" or "grid lines," these terms being understood to include both grids and bus bars as well as any other opaque conductors associated with the light incident side of photovoltaic devices. Use of current-collecting grids is necessary to withdraw power from the photovoltaic device; however, these grids are typically made of high electrical conductivity material such as deposited metal patterns, adhesively adhered metal tapes, metal-containing pastes, metallic inks or plated layers and are quite opaque. The grid lines shade underlying portions of the photovoltaic device thereby preventing it from generating power. Clearly, the grid lines are needed to allow for the efficient withdrawal of photo-generated current, but their presence also detracts from the overall efficiency of the cell. The lines can be made smaller; however, this increases their electrical resistance and thereby tends to decrease cell efficiency. Under the constraints of the prior art, a designer of photovoltaic devices is caught in a dilemma of having to balance the electrical resistance of the cell versus the amount of active area presented for illumination.
In some instances, prior art cells relied upon the use of relatively thin deposits of high conductivity metals such as pure gold, silver, or copper to provide high conductivity, relatively small area grid lines. However, such approaches require the use of sophisticated photolithographic techniques for patterning the grid lines. Additionally, the length of such thin grid lines was limited by the need to avoid high resistivity; consequently, this approach is limited in size and is quite expensive. Lower cost, easier to apply grid lines prepared from paste or ink material are quite desirable; however, they are of lower conductivity and hence must be made fairly thick and wide to achieve sufficient current carrying capabilities. Such materials were not heretofore practical since the grid lines they provide create a high level of shading. What is needed is a method and structure which permits the use of relatively wide grid line patterns, while minimizing shading from those grid lines.
Various attempts have been implemented in the prior art to employ optical systems to concentrate light in areas remote from grid lines. Such approaches involve the use of prismatic arrays and the like. These arrays are typically supported in a spaced-apart relationship with the photovoltaic device or they are adhesively affixed to the light incident side of the device and, when properly aligned, redirect light falling in the region of grid lines to grid-free portions of the device. This technology is typically employed in conjunction with concentrator cells. An overview of this technology is presented by Zhao et al in a paper entitled "Improvements in Silicon Concentrator Cells," published in the Proceedings of the 4th International Photovoltaic Science and Engineering Conference, Sydney, Australia, Feb. 14-17, 1989, Vol. 2, p. 581. Use of a Fresnel, lenticular concentrator is also disclosed in U.S. Pat. No. 4,711,972. The use of grooved top covers is also disclosed in U.S. Pat. Nos. 4,379,202 and 4,453,030. Moreover, the '202 patent discloses an embodiment with the grooves formed in a bottom face of the cover (FIGS. 6 & 7). However, the disclosure cautions that the grooves must define volumes having a lower index of refraction than does the cover.
U.S. Pat. No. 5,110,370 discloses a photovoltaic cell and method of manufacture, the cell including a light directing optical element integrally formed on the top surface of an encapsulant layer in the region of opaque, current-collecting grid lines. The optical elements consist of V-shaped grooves formed in the top surface of the encapsulant layer (preferably by embossing with a die). The geometry of the grooves redirects incident light falling thereon onto adjacent regions of the cell, thereby decreasing the shading affects described above.
The aforementioned patent discloses grid lines which are preferably fabricated from an electrically-conductive ink or paste or of a metallic body adhered to the top electrode layer. In some instances, however, it would be useful to form the grid lines of electrically conductive wire, which would provide good electrical conductivity and low resistance at a low cost. However, the geometry of cylindrical wire tends to exacerbate the shading problems noted above; in addition it is difficult to achieve good electrical contact between the wire and the top electrode since only a very small portion of the cylindrical surface of the wire actually contacts the electrode. Clearly, it would be advantageous to provide a device which utilizes conductive wire as the electrically conductive grid line, yet which also minimizes the shading problems otherwise caused by such wire.
It would also be desirable to provide a photovoltaic cell which takes full advantage of the optical properties of reflective surfaces to minimize the efficiency losses caused by the grid line structure, yet does not require filling in grooves with a low refractive index encapsulant.